A lithographic printing plate precursor has a structure such that an image-forming composition is provided on a substrate. In a typical manufacturing process therefor, an image-forming composition dispersed or dissolved in an organic solvent and, if desired, an upper layer such as protective layer are coated and dried on a surface of a support appropriately subjected to surface treatment, undercoating, back coating and the like. In a typical plate-making process, physical properties of an image-forming composition on a support are changed imagewise by the surface exposure of contact or projection system through an image mask, or by the direct exposure using scanning or modulation of an electromagnetic wave based on image information from a computer, and thereafter, the image-forming composition in the non-image area is removed (developed), if desired, followed by a treatment for providing hydrophilic state, ink-receptive state or forming a protective film, whereby a lithographic printing plate precursor having a non-image area comprising a hydrophilic support surface layer and an image area comprising a hydrophobic composition surface layer is obtained. In a typical printing process of the lithographic printing plate obtained as such, the hydrophilic non-image area receives a fountain solution and the lipophilic image area receives an ink, whereby an ink image is formed on the surface. The obtained ink image is directly or indirectly transferred to a desired printing medium and thereby, a printed matter is obtained.
With respect to the image-forming layer used herein, various techniques are already known, for example, a technique of causing a negative change from soluble to insoluble or positive change from insoluble to soluble as physical properties on exposure, a technique of using a photochemical reaction or a heat mode process as a principle for the change in physical properties, or a technique of using a heat-sensitive recording system. Whichever image formation layer is used, technical problems to be solved are present in common for realizing a lithographic printing plate precursor having high usefulness. Namely, the technical problems are (1) that the image-forming layer has high uniformity and (2) that the image area has high hydrophobicity and the non-image area has high development removability. The uniformity of the image area is mainly related to the above-described production process in view of technique. A plate precursor insufficient in the uniformity seriously reduces the basic performance required for the printing plate such that a large number of homogeneous printed matters having a high-quality image are stably provided, and this plate precursor is not preferred. The high hydrophobicity of the image area is very important from the standpoint of elevating the durability against developer in the plate-making process and thereby obtaining excellent resolution or obtaining a sufficiently long press line or a sufficiently high inking property in the printing process. However, the high hydrophobicity of the image area may cause reduction of the solubility in an alkali aqueous solution usually used as a developer and in turn, undesirable results may be brought about, such as development failure of the non-image area or generation of sludge ingredient in the developer. In other words, the hydrophobicity of the image area and the removability of the non-image area are simply conflicting properties required for the image-forming layer, and a technique capable of achieving these properties at the same time is difficult to develop but is an important matter.
Regarding to this technical problem, a composition containing a polymer compound having a fluoroaliphatic group is known to be useful as an image-forming composition. For example, JP-A-54-135004 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses this composition as an effective technique for improving the uniformity of the image-forming layer. JP-A-62-170950, JP-A-8-15858 and JP-A-2000-19724 disclose a copolymer containing a monomer unit having a fluoroaliphatic group and a monomer unit having a specific functional group, with its usefulness. In these techniques, the deficient point in prior techniques disclosing the use of a polymer compound having a fluoroaliphatic group is improved merely by selecting an additional substituent capable of reducing the adverse effect of the polymer having a fluoroaliphatic group on the plate-making and printing processes or the other way effectively utilizing the polymer. To speak specifically, JP-A-62-170950 discloses an effect of more improving the function of bringing out the layer uniformity, resulting from the improvement of surface activation, JP-A-8-15858 discloses an effect of eliminating the retardation of developability ascribable to the hydrophobicity and JP-A-2000-19724 discloses an effect of forming a high-contrast image by utilizing the hydrophobicity and the orientation force and thereby achieving the hydrophobicity of the image area and the removability of the non-image area. Among these effects of the polymer compound having a fluoroaliphatic group, the layer uniformity is considered to come out by virtue of the surface activation capability of the polymer compound having a fluoroaliphatic group, in other words, a capability of reducing the surface tension of an organic solvent dispersion solution of image-forming composition in the production process. Other effects of the polymer compound having a fluoroaliphatic group are considered attributable to the high hydrophobicity of the fluoroaliphatic group-containing polymer compound contained in the image-forming layer of a lithographic printing plate precursor and also to its capability of orienting, deviating or localizing toward the surface of the image-forming layer. That is, the fluoroaliphatic group-containing polymer compound distribution in the image-forming layer can be made relatively high in the vicinity of the surface and therefore, high hydrophobicity can be imparted particularly to the surface while maintaining the development removability of the photosensitive layer as a whole. For improving the polymer compound having a fluoroaliphatic group, means other than the selection of copolymerization components may also be used. For example, JP-A-2000-187318 discloses a technique where an image-forming material having excellent discrimination in solubility between the image area and the non-image area can be obtained by a polymer compound using a monomer unit having two or more fluoroaliphatic groups.
As such, when the image-forming layer containing a fluoroaliphatic compound is used for a lithographic printing plate precursor, this image-forming layer is effective means for solving the above-described technical problems (1) and (2). However, the effects thereof are not sufficiently high and in fact, more improvements are demanded.
For example, in the case of using a positive image-forming layer, for obtaining a good printed matter, the image obtained after exposure and development preferably has a discrimination as high as possible between the image area and the non-image area, namely, a gradation (contrast) as high as possible, in view of image reproducibility and scratch resistance. At the same time, the requirements for high sensitivity, no blurring, white light safety and wide development latitude must be satisfied, however, a technique capable of fully satisfying these requirements has not yet been developed.
The term “soft image” as used herein means that when an image is exposed through a step wedge and then developed, the difference between the step number where the image starts remaining and the step number where the layers completely remain is large. The term “high-contrast image” as used herein means that the difference between the step number where the image starts remaining and the step number where the layers completely remain is small.
The “blurring” occurs when a lith film floats up due to a gas generated by the decomposition of a photosensitive substance and a complete contact exposure cannot be performed. In general, as the image has higher contrast with equal clear sensitivity, the blurring can be more easily eliminated. The “white light safety” indicates the safety of sensitivity of an image against exposure of the printing plate to white light such as fluorescent lamp and as the image has higher contrast, the white light safety is higher. The “step wedge” is a strip film having steps changed in the density with a variation of 0.15 one by one and this film is used for determining the relationship between the exposure amount and the amount of photosensitive layers remaining after the exposure and development. The “clear sensitivity” means the sensitivity when an image starts to form after the exposure and development. The “development latitude” is used for examining how much the image sensitivity is changed after the exposure and development with the change in the concentration of the developer and as the change in sensitivity is smaller, the development latitude is higher.
In a photopolymerization-type printing plate containing a polymerizable monomer having a double bond and a photopolymerization initiator as a representative negative lithographic printing plate, particularly a laser direct exposure-type printing plate having high sensitivity to a laser beam in the visible light region, the gradation thereof is conventionally soft and therefore, if a printing plate is fixed and the image exposure is performed by an inner drum-type laser plate setter which performs the exposure while rotating a mirror at a high speed, fogging is readily generated due to scattered or reflected light. In order to elevate the impression capacity of the printing plate, it is preferred to perform the exposure with a high energy, however, since fogging due to scattered or reflected light goes for the worth, the exposure amount cannot be increased to elevate the impression capacity. The impression capacity must be elevated without causing any generation of fogging due to scattered or reflected light even at a high exposure amount. This problem may be overcome by creating a high-contrast graduation. In the image exposure by a laser, the exposure time is about 1μ second order per 1 dot and fogging due to scattered or reflected light occurs when a photosensitive material is exposed to extremely weak light for a long period of time such as a few minutes order and the photosensitive layer is thereby photocured. When the gradation of a photosensitive material is contrasted, the photosensitive material is hardly photocured by the weak light and can be removed by the development, as a result, fogging does not occur. In the case of a heat-sensitive lithographic printing plate on which the drawing is performed using an infrared laser, the discrimination between the image area and the non-image area is low, that is, the graduation is low (soft), and therefore, the portion touched with a hand may undergo slipping of an image or suffer from bad stability against external scratching.